1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, and an updating method of a multi-dimensional LUT.
2. Description of Related Art
In an image forming apparatus, optimal reproduction of colors is achieved through procedures such as primary color calibration, multi-color calibration, and update of a multi-dimensional LUT called a color profile.
Primary color calibration is a process of updating γ correction curves for cyan (C), magenta (M), yellow (Y), and black (K) colors which are used in image formation, and achieves stable color reproducibility regardless of variable printing characteristics of the image forming apparatus. Multi-color calibration is a process of updating γ correction curves for mixed colors that are mixtures of at least two of the C, M, Y, and K colors. The γ correction curves for primary colors and multi-colors are stored in a one-dimensional LUT.
A multi-dimensional LUT which is used for color conversion receives, for example, four-dimensional CMYK values and, in response, outputs four-dimensional CMYK values corresponding to the printing characteristics of the image forming apparatus.
A new one-dimensional LUT and a new multi-dimensional LUT can be prepared by forming single-color or mixed-color patches of different tones and performing colorimetry on the patches.
The update of the multi-dimensional LUT achieves the highest color reproducibility. However, the update of a multi-dimensional LUT requires colorimetry on patches of at least one thousand mixed-colors with a prolonged operation time, whereas in the primary-color or multi-color calibration, colorimetry is performed on a small number of patches and calibration can be readily performed. It is preferable to update a multi-dimensional LUT at a minimum frequency because the productivity of the image forming apparatus decreases during the updating process.
A multi-dimensional LUT must be updated if the reproducibility of mixed colors varies greatly to an extent that color shades cannot be reproduced even though the targeted color reproducibility has been achieved for the primary colors (single colors). A variation in the color reproducibility that requires the update of the multi-dimensional LUT is often visually determined by a user observing a printed material or multi-color patches prepared during primary-color or multi-color calibration.
Such visual determination, however, is a difficult and complicated process. Moreover, the update of the multi-dimensional LUT based on such visual determination, which is a subjective process, may not be appropriate.
As a prerequisite, the color reproducibility must be the same for the primary colors. Thus, the multi-dimensional LUT may be automatically updated in cooperation with the primary-color calibration (refer to Japanese Patent Application Laid-Open No. 2010-217673 and No. 2005-165464).
A variation in the color reproducibility for the primary colors, however, does not necessarily lead to a variation in the multi-dimensional LUT. Thus, the update of the multi-dimensional LUT is not required in such a case.
According to the above former Patent document, the primary calibration is determined on the basis of single-color patches, but necessity of updating the multi-dimensional LUT is not determined. Similarly, according to the latter patent document, the adequacy of the data read from the patches to be used for the update of the multi-dimensional LUT is confirmed, but necessity of updating the multi-dimensional LUT cannot be determined.
In another case, a user may be satisfied with the current color reproducibility and may want to maintain the current multi-dimensional LUT even if the color reproducibility of the multi-dimensional LUT varies from the targeted color reproducibility. Hence, the update of the multi-dimensional LUT in accordance with the variation in the color reproducibility does not always meet the needs of the user.
A plurality of image forming apparatuses may be connected to constitute an image forming system. For example, a plurality of image forming apparatuses may be connected in series to constitute a so-called tandem image forming system or a plurality of image forming apparatuses may be connected via a network to constitute an image forming system. Ideally, every image forming apparatus in such a system should have the same color reproducibility; therefore, the image forming apparatuses are intended to have common color reproducibility (for example, refer to Japanese Patent Application Laid-Open No. 2007-137013).
In the above patent document, the color gamut of the image forming apparatuses is compressed and the color reproducibility gamut is shared among the image forming apparatuses. Unfortunately, the gradations of mixed colors may not match in the compressed color gamut. Optimal color reproducibility is achieved by updating the multi-dimensional LUTs of the image forming apparatuses to a common multi-dimensional LUT. In such a case, also, necessity of updating multi-dimensional LUT should be determined appropriately.
An object of the present invention is to readily and appropriately determine necessity of updating the multi-dimensional LUT.
In order to achieve the above object, according to the first aspect of a preferred embodiment of the present invention, there is provided an image forming apparatus including an image forming unit which forms an image on a sheet by using color materials of a plurality of colors on the basis of image data; a color conversion unit which converts a color space of the image data to a color space of the color materials used by the image forming unit by using a multi-dimensional LUT; a storage unit which stores a mixed-color pattern including a gradation of tones of a mixed color and a reference value of each of the tones, the mixed color made of at least two colors among the plurality of colors; and a control unit which forms an image of the mixed-color pattern on the sheet with the image forming unit, compares color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern formed on the sheet and the reference value of the mixed-color pattern stored in the storage unit, and determines that the multi-dimensional LUT needs to be updated and updates the multi-dimensional LUT if a difference of the color values is determined to be large on the basis of a threshold value.
According to the second aspect of a preferred embodiment of the present invention, there is provided an image forming apparatus including an image forming unit which forms an image on a sheet by using color materials of a plurality of colors on the basis of image data; a color conversion unit which converts a color space of the image data to a color space of the color materials used by the image forming unit by using a multi-dimensional LUT; a storage unit which stores a mixed-color pattern including a gradation of tones of a mixed color and a reference value of each of the tones, the mixed color made of at least two colors among the plurality of colors; and a control unit which forms an image of the mixed-color pattern on the sheet with the image forming unit, compares color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern formed on the sheet and the reference value of the mixed-color pattern stored in the storage unit, and updates the reference value of the mixed-color pattern stored in the storage unit to the observed value for each of the tones and maintains the current multi-dimensional LUT if the difference of the color values is determined to be large on the basis of a threshold value.
According to the third aspect of a preferred embodiment of the present invention, there is provided an image forming system including a plurality of image forming apparatuses which are connected in series including a first image forming apparatus serving as a reference and a second image forming apparatus not serving as a reference, each of the image forming apparatuses including an image forming unit which forms an image on a sheet by using color materials of a plurality of colors on the basis of image data; a color conversion unit which converts a color space of the image data to a color space of the color materials used by the image forming unit by using a multi-dimensional LUT; a storage unit which stores a mixed-color pattern including a gradation of tones of a mixed color and a reference value of each of the tones, the mixed color made of at least two colors among the plurality of colors; and a control unit which updates the multi-dimensional LUT, wherein, each of the image forming apparatuses forms an image of the mixed-color pattern on the sheet, and the second image forming apparatus compares color values representing color shades for each of the tones between an observed value of the mixed-color pattern provided by the first image forming apparatus, which is a reference value, and an observed value of the mixed-color pattern provided by the second image forming apparatus, and determines that the multi-dimensional LUT of the second image forming apparatus needs to be updated and updates the multi-dimensional LUT if the difference of the color values is determined to be large on the basis of a threshold value.
According to the fourth aspect of a preferred embodiment of the present invention, there is provided an image forming system including a plurality of image forming apparatuses connected via a network, each of the image forming apparatuses including an image forming unit which forms an image on a sheet by using color materials of a plurality of colors on the basis of image data; a color conversion unit which converts a color space of the image data to a color space of the color materials used by the image forming unit by using a multi-dimensional LUT; a storage unit which stores a mixed-color pattern including a gradation of tones of a mixed color and a reference value of each of the tones, the mixed color made of at least two colors among the plurality of colors; and a control unit which updates the multi-dimensional LUT, wherein, each of the image forming apparatuses forms an image of the mixed-color pattern on the sheet, compares color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern formed on the sheet and the reference value of the mixed-color pattern stored in the storage unit, and determines that the multi-dimensional LUT needs to be updated and updates the multi-dimensional LUT if the difference of the color values is determined to be large on the basis of a threshold value.
According to the fifth aspect of a preferred embodiment of the present invention, there is provided a method of updating a multi-dimensional LUT which is used by an image forming apparatus to convert a color space of image data to another color space of color materials used in image formation, the method including forming an image of a mixed-color pattern which includes a gradation of tones of a mixed color made of at least two colors on a sheet; determining whether a difference of the color values is large on the basis of a threshold value by comparing color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern on the sheet and a reference value of the mixed-color pattern; and updating the multi-dimensional LUT by determining that the multi-dimensional LUT needs to be updated if the difference is determined to be large.
According to the sixth aspect of a preferred embodiment of the present invention, there is provided a method of updating a multi-dimensional LUT which is used by an image forming apparatus to convert a color space of image data to another color space of color materials used in image formation, the method including forming an image of a mixed-color pattern which includes a gradation of tones of a mixed color made of at least two colors on a sheet; determining whether a difference of the color values is large on the basis of a threshold value by comparing color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern on the sheet and a reference value of the mixed-color pattern stored in the image forming apparatus; and updating the color value of the reference value of the mixed-color pattern stored in the image forming apparatus to the observed value for each of the tones and maintaining the current multi-dimensional LUT if the difference is determined to be large.
According to the seventh aspect of a preferred embodiment of the present invention, there is provided a method of updating a multi-dimensional LUT to convert a color space of image data to a color space of color materials used in image formation by a plurality of image forming apparatuses which are connected in series and include at least a first image forming apparatus serving as a reference and a second image forming apparatus not serving as a reference, the method including forming an image of a mixed-color pattern which includes a gradation of tones of a mixed color made of at least two colors on a sheet by each of the image forming apparatuses; determining whether the difference of the color values is large on the basis of a threshold value in the second image forming apparatus by comparing color values representing color shades for each of the tones between an observed value of the mixed-color pattern formed by the first image forming apparatus and an observed value of the mixed-color pattern formed by the second image forming apparatus; and updating the multi-dimensional LUT in the second image forming apparatus by determining that the multi-dimensional LUT of the second image forming apparatus needs to be updated if the difference is determined to be large.
According to the eighth aspect of a preferred embodiment of the present invention, there is provided a method of updating a multi-dimensional LUT to convert a color space of image data to another color space of color materials for image formation used by a plurality of image forming apparatuses connected via a network, the method including forming an image of a mixed-color pattern including a gradation of tones for a mixed color made of at least two colors on a sheet by each of the image forming apparatuses; determining whether the difference of the color values is large on the basis of a threshold value in each of the image forming apparatuses by comparing color values representing color shades for each of the tones between an observed value of the mixed-color pattern formed on the sheet and a reference value of the mixed-color pattern; and determining that the multi-dimensional LUT needs to be updated and updating the multi-dimensional LUT by each of the image forming apparatuses if the difference is determined to be large.